In the conventional art, there is known a machine tool in which machining such as cutting is performed by moving a tool relative to a workpiece. A numerical control type machine tool is also known in which, in such a machine tool, a tool path is specified by the coordinate or the like of a predetermined feed axis, and machining is performed while the tool is being moved with respect to the workpiece. The machine tool can perform machining automatically while changing the relative position of the tool with respect to the workpiece by movement of at least one of the workpiece and the tool in accordance with a command from a control apparatus.
During a period in which the workpiece is machined by the machine tool, the tool moves relative to the workpiece in order to remove a part of the workpiece. A tool such as an end mill performs machining while rotating about its axis. During the machining of the workpiece, a force is applied to the tool. For example, the tool is applied with a force in a direction opposite to the direction that the tool travels with respect to the workpiece. Furthermore, the rotating tool is applied with a force in a direction opposite to the direction in which the tool rotates. When an excessive force is applied to the tool during machining of the workpiece, there is a possibility that the tool may be damaged or the spindle device may break down. For example, the tool may be broken, or an edge of the tool may be chipped.
In Japanese Unexamined Patent Publication No. 2005-205517A, there is disclosed a cutting control method for a machine tool in which it is monitored whether or not the spindle load during cutting of a workpiece falls within a target load range. In this cutting control method, it is disclosed that when the spindle load exceeds the target load range, each override value with respect to each speed setting value of the spindle rotation speed and the cutting feed speed is decreased so as to control the rotational speed of the spindle and the cutting feed speed.